The present invention relates to forming metal parts, and in particular, to an improved method of forming Titanium part blanks over dies under superplastic conditions in order to avoid thinning or necking in the formed parts.
For many years it has been known that certain metals, such as Titanium, as well as certain metal alloys, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus when in a superplastic condition, the metal or metal alloy exhibits low resistance to deformation and may be elongated with controlled thinning. This permits a sheet of such metal to be readily formed against dies to achieve desired shapes. Superplastic forming (SPF) may be performed in conjunction with diffusion bonding. Diffusion bonding refers to metallurgical joining of surfaces of similar or dissimilar metals by holding them in physical contact and applying heat and pressure sufficient to cause commingling of the atoms at the junction. Further details of both SPF and diffusion bonding may be had by way of reference to U.S. Pat. No. 3,934,441 of Hamilton et al. entitled "Controlled Environment Superplastic Forming of Metals" and U.S. Pat. No. 3,927,817 of Hamilton et al. entitled "Method of Making Metallic Sandwich Structures".
Figs. 1a and 1b illustrate an older conventional technique of SPF which is known as diaphragm forming. Referring to Fig. 1a, a relatively large sheet of Titanium 10 is laid horizontally across an upwardly opening steel forming chamber 12. The chamber is supported in a hydraulic press (not shown) so that a steel cover 14 can be closed against the chamber from above. The peripheral edges of the Titanium sheet are firmly clamped between the mating edges: the forming chamber and cover which has a peripheral seal (not visible). The sheet is then heated to the appropriate temperature and formed around a ceramic die 16 supported in the forming chamber as illustrated in Fig. 1b. This formation results from the introduction of pressurized Argon gas on both sides of the sheet and the subsequent release of pressurized gas on the lower side of the sheet. See for example U.S. Pat. No. 3,974,673 of Fosness et al. entitled "Titanium Parts Manufacturing" wherein a smaller radiation shield 64 is also draped over the sheet directly above the die.
The diaphragm forming approach is not compatible with dies having relatively large vertical dimensions. This is because thinning of the sheet in proportion to the amount and depth of forming is inherent. The peripheral seal prevents inward slippage of the sheet edges and thinning of the sheet occurs where large deformations are necessary, resulting in weak points in the formed part. With such dies uniform thicknesses could be achieved if the edges of the metal sheet were horizontally drawn in to accommodate substantial downward stretching of the sheet. However, if the seal is eliminated and the clamp pressure is lessened so that sheet can slide between the cover and chamber, it is not possible to maintain the desired gas pressures.
FIGS. 2a and 2b illustrate a newer technique of SPF I developed which is known as drape forming. It has been used successfully on a commercial basis for several years to form parts around dies that extend substantial distances in a direction normal to the initial plane of the Titanium sheet so that there is little or no thinning or necking. A relatively smaller Titanium sheet 18 which is the part blank is positioned directly over the die 16 and is driven against and around the die by the overlying relatively larger driver sheet 10. The edges of the part blank 18 are free to pull inwardly to thereby alleviate any thinning that would otherwise occur. Typically a Boron Nitride powder is used to facilitate sliding contact between the underside of the part blank 18 and the outer surface of the ceramic die 16. See for example U.S. Pat. No. 4,269,053 of Agrawal et al. entitled "Method of Superplastic Forming Using Release Coatings with Different Coefficients of Friction". A similar release compound may also be used between the driver sheet and part blank to reduce friction therebetween.
In the drape forming approach, the part blank may be tack or spot welded to the driver sheet at appropriate locations when both are still flat in order to maintain the proper positioning of the part blank over the die during the SPF process. The positions of the welds are chosen so that they do not inhibit drawing in of the edges of the part blank as necessary to prevent thinning. Alternatively, the flat part blank may be cut to provide arms which extend therefrom and contact the side walls to hold the part blank in position over the die during the SPF process. Such positioning arms may also be separate pieces spot welded to the part blank. The driver sheet may be made of Titanium or mild steel, and where the former is utilized, it may be less expensive low grade or reject Titanium sheet which itself cannot be used to form an aircraft part. The part blank may be formed with tabs to provide a gripping surface to aid in separating the formed part from the complementary formed driver sheet. One formed driver sheet can be used as an apply template to designate part trim and hole locations, for all subsequent parts.
Heretofore the forming chamber which has been used in drape forming has been made entirely of steel. Because of the high temperatures involved in SPF, e.g. 1600-1700 degrees F., the bottom wall of the steel chamber has had a tendency to bow, which sometimes results in fracturing of the ceramic die supported thereon. The chamber must be made of Chrome-Nickel steel to withstand the high temperatures, but still it ends up having a limited life. Replacing the chamber is both time consuming and costly. Also, the steel chamber has relatively thick walls, and therefore a relatively large mass. This mass is heated by resistance type electric heaters in the SPF press. The outer walls of the steel chamber are thermally insulated with a water cooled jacket. Nevertheless, each time the press is opened a tremendous amount of heat is lost, resulting in substantial additional electric power being consumed in order to maintain the high temperatures required.